1. Technical Field
The present invention is directed to a nickel coating that is resistant to cracking and a process for forming such a nickel coating onto an article.
2. Art Background
Devices such as integrated circuits are mechanically and electrically connected to larger assemblies via lead frames. The integrated circuit or other device is mechanically mounted on the lead frame, and the device is then electrically connected to the leads on the lead frame. The lead frame is then electrically and mechanically connected to a larger assembly. After the device is mounted on the lead frame, the device is encapsulated for protection. The process for mounting and electrically interconnecting the device to the lead frame and connecting the lead frame to a larger assembly includes steps for device attachment, cure, wirebonding, encapsulation, trim-and-forming and soldering. Some of these steps subject the metal lead frame to mechanical stress and strain. For example, when the leads of the lead frame are formed according to the industrial standard, the forming angle is about 82 to about 90 degrees and the forming radius is about 250 .mu.m.
Lead frames have been formed from a variety of materials. Lead frame materials are selected for their mechanical strength, conductivity, machinability, formability, corrosion resistance, wirebondability, solderability, and thermal expansion. Although gold and palladium have the. desired characteristics, the cost of these materials makes their use prohibitive for most applications. Copper and copper alloys also have many advantageous properties that make it suited for this application. A number of different copper alloys are used including alloy 151 (99.9 wt. % copper/0.1 wt. % zirconium); alloy 194 (97.5 wt. % copper/ 2.35 wt. % iron/ 0.03 wt. % phosphorous/0.12 wt. % zinc); and alloy 7025 (96.2 wt. % copper/3.0 wt. % nickel/0.65 wt. % silicon/0.15 wt. % magnesium). However, the corrosion of the copper in air and the difficulty of forming good soldered connections to copper create the need to use coated copper lead frames. The coating on the lead frame provides corrosion protection and provides a good solderable surface. An iron-nickel alloy, alloy 42 (58 wt. % iron and 42 wt. % nickel), also has properties that make it useful as a lead frame. However, the corrosion of this metal in air also precludes the use of uncoated alloy 42 as a lead frame material.
Typically, the copper and iron containing materials are coated with nickel to prevent the oxidation of the underlying copper or iron. However, nickel also oxidizes in air, and such oxides are undesirable. A thin layer of a metal that does not oxidize is plated over the nickel to prevent these oxides from forming. Examples of these materials, typically referred to as "noble" metals, include silver, palladium, and gold. These thin coatings range in thickness from about 0.025 .mu.m to about 1.5 .mu.m.
Nickel coatings applied using conventional electrodeposition techniques have a tendency to crack when the lead frame is subjected to the stresses and strains associated with the trim-and-form steps that are discussed above. When the nickel layer cracks, the layer of noble metal thereon also cracks. When these metal coatings crack, the underlying copper or iron alloy oxidizes, corrodes, and migrates to the surface, in the presence of humidity. These surface deposits have an adverse effect on the packaged device. Consequently, a nickel coating for a lead frame that does not crack when the lead frame is subjected to stresses and strains associated with the packaging of electronic devices is required.